This invention relates to high voltage power supplies such as those used in plasma sputtering, space electric propulsion, lasers and X-ray machines, as well as to power supplies that have a wide full-power operating range that allows them to provide high current at low output voltages and low current at high output voltages.
U.S. Pat. No. 6,697,265 teaches multiphase L-C-C resonant power supplies for plasma sputtering applications having a wide full-power output voltage range. These power supplies have capacitors in parallel with output rectifier diodes, with at least two diode-capacitor combinations connected in series between the output voltage terminals. The output rectifier circuit shown in FIG. 5 of that patent can produce a maximum dc voltage of twice the peak value of the rectifier ac input voltage. The voltage for the output rectifier is typically supplied from a transformer. In high-voltage power supplies, it can be advantageous to reduce the voltage that the transformer is required to provide by using diode-capacitor multiplier circuits such as Cockcroft-Walton multipliers to increase the ratio of the dc output voltage to the ac voltage supplied by the transformer. The rectifier circuit of FIG. 5 of U.S. Pat. No. 6,697,265 also requires an output inductor to reduce ripple and increase the full-power output voltage range. Inductors have significant mass, and removing them is particularly advantageous for space applications. The output rectifier circuit shown in FIG. 7 of that patent does not require an output inductor, but the peak dc output voltage is only 1.15 times the peak ac input voltage of the rectifier.
U.S. Pat. No. 4,994,953 teaches the use of voltage multipliers to rectify the output voltage of three-phase inverters for space applications, but the output current of voltage multipliers is limited by the driving capacitors that supply the ac current to the rectifier diodes. This current limiting effect is described in U.S. Pat. No. 3,381,204 in terms of a capacitor reactance voltage drop. The output current limitation prevents the power supply from operating over a wide full-power operating voltage range unless impractically large driving capacitors were used.
U.S. Pat. No. 7,477,042 utilizes a set of three voltage multipliers identical to the teachings of U.S. Pat. No. 4,994,953, but it explicitly describes a string of filter capacitors that is shared among the three voltage multipliers. Significant energy is stored in this string of filter capacitors. Plasma loads such as those used in sputtering and electric propulsion can develop low impedance arcs that are detrimental if too much energy is dumped into them. Consequently, power supplies with low stored energy are preferred. The common string of filter capacitors described in these prior art patents can have less capacitance than three separate voltage multipliers with three separate filter capacitor strings due to the reduction of currents flowing in the filter capacitor string at common nodes when the currents from the three phases are combined. What was not recognized in the prior art, however, was that the common string of filter capacitors may be entirely removed, thereby further reducing the stored energy.
The use of multiple switching power supplies with staggered switching phases for powering Hall effect thrusters for electric propulsion is described in Button, R. M. et al, “Digital control technologies for modular DC-DC converters,” IEEE 2000 Aerospace Conference Proceedings, Volume 5, 18-25 Mar. 2000 pp. 355-362. However, the converters described in this reference utilize filter inductors that add to the mass.
F. Belloni et al., “Parameters Optimization for Improved Dynamics of Voltage Multipliers for Space,” IEEE PESC 2004, pp. 439-433 teaches that the driving capacitors in voltage multipliers may have values that are progressively reduced in successive stages in order to decrease the output impedance and improve the transient response, but staggering the capacitance values in a multiplier circuit to widen the full-power operating range is not discussed.
The dc power supply of the present invention provides dc power over a wide range of output voltages at full operating power by utilizing multiplier circuits supplied by a source of high-frequency alternating current. These multiplier circuits include a plurality of multiplier cells, each containing series-connected diodes and a driving capacitor. The multiplier cells are shunted by bypass rectifiers that are arranged to allow current to flow from multiplier input terminals to output terminals of the power supply. The bypass rectifiers do not conduct current for low output current levels, but conduct increasing levels of current when output currents increase beyond a conduction threshold value, thereby increasing the maximum available output current. The multiplier driving capacitors exhibit significant ripple voltages at moderate output currents, and may be completely discharged each cycle at higher output currents. Interactions among the diodes and capacitors in the multiplier circuits cause the amplitude of the alternating currents delivered by the source of high-frequency alternating current to the Multiplier input terminals to remain relatively constant as the output voltage is varied over a wide range while operating at full power. This allows the efficiency of the power supply to remain high over a wide range of output voltages.